all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/connections/tcp_dial.go

@@ -62,6 +62,7 @@ func (d *tcpDialer) Dial(ctx context.Context, _ protocol.DeviceID, uri *url.URL)
 	if isLocal {
 		priority = d.lanPriority
 	}
+
 	return newInternalConn(tc, connTypeTCPClient, isLocal, priority), nil
 }
 
@@ -73,9 +74,10 @@ func (tcpDialerFactory) New(opts config.OptionsConfiguration, tlsCfg *tls.Config
 			trafficClass:      opts.TrafficClass,
 			reconnectInterval: time.Duration(opts.ReconnectIntervalS) * time.Second,
 			tlsCfg:            tlsCfg,
+			lanChecker:        lanChecker,
 			lanPriority:       opts.ConnectionPriorityTCPLAN,
 			wanPriority:       opts.ConnectionPriorityTCPWAN,
-			lanChecker:        lanChecker,
+			allowsMultiConns:  true,
 		},
 		registry: registry,
 	}